Method of manufacturing an almgsc-series alloy product

ABSTRACT

The invention relates to a method of manufacturing an AIMgSc-series aluminium alloy product, the method comprising the step of cooling said AIMgSc-series aluminium alloy product from a final annealing temperature to below 150° C., wherein the cooling in a first temperature range of about 250° C. to about 200° C. is at an equivalent time of more than 4 hours, and wherein the cooling in a second temperature range from about 200° C. to about 150° C. is at an equivalent time of more than 0.2 hours, and wherein the equivalent time (t(eq)) is defined as (I) wherein T (in degrees Kelvin) indicates the temperature of the heat treatment, which changes over the time t (in hours) and T ref  (in degrees Kelvin) is the reference temperature selected at 473K. 
     
       
         
           
             
               
                 
                   
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FIELD OF THE INVENTION

The invention relates to a method of manufacturing an AlMgSc-seriesaluminium alloy product. The resultant product provides an improvedcorrosion resistance.

The aluminium alloy product can be in the form of a rolled product(sheet or plate), an extruded product, a forged product or apowder-metallurgy product.

BACKGROUND OF THE INVENTION

AlMg-series aluminium alloys which may optionally or mandatorily have Scas alloying element are known in the art, for example from the followingdocuments:

U.S. Pat. No. 6,695,935-B1 (Corus/Aleris) discloses an alloy in the formof a rolled or extruded product and having the composition of: 3.5-6.0%Mg, 0.4-1.2% Mn, 0.4-1.5% Zn, up to 0.25% Zr, up to 0.3% Cr, up to 0.2%Ti, up to 0.5% Fe, up to 0.5% Si, up to 0.4% Cu, one or more selectedfrom the group of (0.005-0.1% Bi, 0.005-0.1% Pb, 0.01-0.1% Sn, 0.01-0.5%Ag, 0.01-0.5% Sc, 0.01-0.5% Li, 0.01-0.3% V, 0.01-0.3% Ce, 0.01-0.3% Y,0.01-0.3% Ni), others each 0.05%, total 0.15%, balance aluminium. Thealloy is said to offer improved long-term corrosion resistance in bothsoft temper (O-temper) and work- or strain-hardened temper (H-temper) ascompared to those of the standard AA5454 alloy.

EP-1917373-B1 (Aleris) discloses an aluminium alloy product having3.5-6.0% Mg, 0.4-1.2% Mn, up to 0.5% Fe, up to 0.5% Si, up to 0.15% Cu,0.05-0.25% Zr, 0.03-0.15% Cr, 0.03-0.2% Ti, 0.1-0.3% Sc, up to 1.7% Zn,up to 0.4% Ag, up to 0.4% Li, optionally one or more dispersoid-formingelements selected from the group consisting of (Er, Y, Hf, V) each up to0.5%, impurities or incidental elements each <0.05%, total <0.15%, andthe balance aluminium.

RU-2280705-C1 discloses an alloy containing 4.2-6.5% Mg, 0.5-1.2% Mn, upto 0.2% Zn, up to 0.2% Cr, up to 0.15% Ti, up to 0.25% Si, up to 0.30%Fe, up to 0.1% Cu, 0.05-0.3% Zr, at least one element selected from thegroup consisting of (0.05-0.3% Sc, 0.0001-0.01% Be, 0.001-0.1% Y,0.001-0.1% Nd, 0.001-0.1% Ce), balance aluminium. The aluminium alloy issaid to have improved ballistic properties. The Zn and Si content arereduced to improve the weldability and to increase the corrosionresistance of the aluminium alloy.

RU-2268319-C1 discloses an alloy containing 5.5-6.5% Mg, 0.10-0.20% Sc,0.5-1.0% Mn, 0.10-0.25% Cr, 0.05-0.20% Zr, 0.02-0.15% Ti, 0.1-1.0% Zn,0.003-0.015% B, 0.0002-0.005% Be, balance aluminium, and wherein the sumof Sc+Mn+Cr is at least 0.85%.

WO-01/12869-A (Kaiser Aluminum) discloses an alloy comprising 4.0-8.0%Mg, 0.05-0.6% Sc, 0.1-0.8% Mn, 0.5-2.0% Cu or Zn, 0.05-0.20% Hf or Zr,and the balance aluminium and incidental impurities.

WO-98/35068 (Alcoa) discloses an aluminium alloy product comprising 3-7%Mg, 0.03-0.2% Zr, 0.2-1.2% Mn, up to 0.15% Si, and 0.05-0.5% of adispersoid-forming element selected from the group consisting of (Sc,Er, Y, Ga, Ho, Hf), the balance being aluminium and incidental elementsand impurities, and wherein the aluminium alloy product is preferablyZn-free and Li-free.

WO-2018/073533-A1 (Constellium) discloses a method for producing ahot-worked product, in particular a sheet product having a thickness ofless than 12 mm, made of an aluminium alloy composed, of Mg: 3.8-4.2%,Mn: 0.3-0.8%, Sc: 0.1-0.3%, Zn: 0.1-0.4%, Ti: 0.01-0.05%, Zr:0.07-0.15%, Cr: <0.01%, Fe: <0.15%, Si<0.1%, wherein the homogenisationis carried out at a temperature of between 370° C. and 450° C., forbetween 2 and 50 hours, such that the equivalent time at 400° C. isbetween 5 and 100 hours, and the hot deformation is carried out at aninitial temperature of between 350° C. and 450° C. The products are saidto be advantageous as they offer a better compromise in terms ofmechanical strength, toughness and hot-formability.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated,aluminium alloy and temper designations refer to the AluminiumAssociation designations in Aluminum Standards and Data and theRegistration Records, as published by the Aluminium Association in 2018and are well known to the persons skilled in the art.

For any description of alloy compositions or preferred alloycompositions, all references to percentages are by weight-percent unlessotherwise indicated.

The term “up to” and “up to about”, as employed herein, explicitlyincludes, but is not limited to, the possibility of zero weight-percentof the particular alloying component to which it refers. For example, upto 0.10% Zn may include an alloy having no Zn.

It is an object of the invention to provide a method of manufacturing anAlMgSc-series aluminium alloy product having an improved corrosionperformance.

It is an object of the invention to provide a method of manufacturing anAlMgSc-series aluminium alloy product having an improved exfoliationcorrosion resistance in combination with an improved intergranularcorrosion resistance.

These and other objects and further advantages are met or exceeded bythe present invention providing a method of manufacturing anAlMgSc-series aluminium alloy product, the method comprising the step ofcooling said AlMgSc-series aluminium alloy product from a finalannealing temperature or a set annealing temperature to below 150° C.,wherein the cooling down in a first temperature range of about 250° C.to about 200° C. is at an equivalent time of more than 4 hours,preferably more than 6.5 hours and more preferably more than 26 hours,and wherein the cooling down in a second temperature range from about200° C. to about 150° C. is at an equivalent time of more than 0.2hours, preferably more than 0.4 hours and more preferably more than 0.8hours, and wherein the equivalent time (t(_(eq))) is defined as

${t({eq})} = \frac{\int{{\exp\left( {{- 16000}/T} \right)}{dt}}}{\exp\left( {{- 16000}/T_{ref}} \right)}$

wherein T (in degrees Kelvin) indicates the temperature of the heattreatment, which changes over the time t (in hours) and T_(ref) (indegrees Kelvin) is the reference temperature selected at 473K (200° C.).

The method according to the invention provides AlMgSc-series aluminiumalloy products have a good strength, preferably Rp>200 MPa, incombination with a good corrosion resistance, in particular a goodexfoliation corrosion resistance in combination with a goodintergranular corrosion resistance. The cooling rates applied areeconomical feasible in an industrial environment of manufacturing theAlMgSc-series aluminium alloy products.

The AlMgSc-series aluminium alloy product manufactured in accordancewith the invention are resistant to exfoliation corrosion. “Resistant toexfoliation corrosion” means that the aluminium alloy product passesASTM Standard G66-99 (2013), entitled “Standard Test Method for VisualAssessment of Exfoliation Corrosion Susceptibility of 5XXX SeriesAluminium Alloys (ASSET Test)”. N, PA, PB, PC and PD indicate theresults of the ASSET test, N representing the best result. The aluminiumalloy products manufactured in accordance with the invention achievebefore and after being sensitised a PB result or better.

The AlMgSc-series aluminium alloy product manufactured in accordancewith the invention are also resistant to intergranular corrosion.“Resistant to intergranular corrosion” means that, both before and afterthe AlMgSc-series aluminium alloy has been sensitized, the aluminiumalloy product passes ASTM Standard G67-13, entitled “Standard TestMethod for Determining the Susceptibility to Intergranular Corrosion of5XXX Series Aluminium Alloys by Mass Loss After Exposure to Nitric Acid”(NAMLT Test)”. If the measured mass loss per ASTM G67-13 is not greaterthan 15 mg/cm², then the sample is considered not susceptible tointergranular corrosion. If the mass loss is more than 25 mg/cm², thenthe sample is considered susceptible to intergranular corrosion. If themeasured mass loss is between 15 mg/cm² and 25 mg/cm², then furtherchecks are conducted by microscopy to determine the type and depth ofattack, whereupon one skilled in the art may determine whether there isintergranular corrosion via the microscopy results. The AlMgSc-seriesaluminium alloy products manufactured in accordance with the inventionachieve a measured mass loss per ASTM G67-13 not greater than 15 mg/cm²,both before and after being sensitized. Preferably the measured massloss is not greater than 12 mg/cm², and more preferably not greater than9 mg/cm². “Sensitized” means that the AlMgSc aluminium alloy product hasbeen annealed to a condition representative of at least 20 years ofservice life. For example, the aluminium alloy product may becontinuously exposed to elevated temperature for several days (e.g., atemperature in the range 100° C. to 120° C. for a period of about 7days/168 hours).

The AlMgSc-series aluminium alloy product may realize resistance tostress corrosion cracking and intergranular corrosion as a result of, atleast in part, due to the absence of a continuous film of β-phaseparticles at the grain boundaries. Aluminium alloy products arepolycrystalline. A “grain” is a crystal of the polycrystalline structureof the aluminium alloy, and “grain boundaries” are the boundaries thatconnect the grains of the polycrystalline structure of the aluminiumalloy, “β-phase” is Al₃Mg₂, and “a continuous film of β-phase” meansthat a continuous volume of β-phase particles is present at the majorityof the grain boundaries. The continuity of the β-phase may bedetermined, for example, via microscopy at a suitable resolution (forexample at a magnification of at least 200×).

In accordance with the invention it has been found that a very fastcooling rate, for example by means of quenching from the final annealingtemperature to below 150° C. has an adverse effect on the corrosionresistance of the AlMgSc-series aluminium alloy product, in particularon the corrosion resistance tested according to the NAMLT-test afterbeing sensitized. A slower cooling rate results in an enhancedintergranular corrosion resistance.

For the cooling down from the final annealing temperature to about 150°C., more in particular in the first temperature range of about 250° C.to about 200° C., the equivalent time should be longer than 4 hours,preferably longer than 6.5 hours, more preferably longer than 26 hoursand in the second temperature range of about 200° C. to about 150° C.,the equivalent time should be longer than 0.2 hours, preferably longerthan 0.4 hours, more preferably longer than 0.8 hours. The relative slowcooling rate is important for the precipitation of discontinuous p-phaseparticles at the grain boundaries and to avoid the precipitation of acontinuous film of p-phase particles, both after cooling to ambienttemperature and after the Al—Mg—Sc alloy has been sensitized. Thecooling down is preferably performed in a continuous mode such that themetal temperature is continuously reduced over time.

The cooling down from the final annealing temperature to the firsttemperature range starting at about 250° C. is not critical. Whenemploying the method according to the invention on an industrial scaleit can be useful or convenient to apply about the same cooling rate asfor the first temperature range.

The further cooling down from about 150° C. to below about 85° C. isless critical and can be done at a higher cooling rate to minimize thecoarsening of precipitates. The cooling rate for the cooling down fromabout 85° C. to ambient temperature is not critical.

In an embodiment the AlMgSc-series aluminium alloy product is in a formselected from the group consisting of a rolled product (sheet or plate),an extruded product, a forged product and a powder-metallurgy product.In a further embodiment any of these products are in a welded conditionor in a formed condition.

In a particular embodiment the AlMgSc-series aluminium alloy product isin the form of a rolled product. In a further embodiment the rolledproduct has been welded or formed.

In an embodiment the thickness of the AlMgSc-series aluminium alloyrolled product is at most 25.4 mm (1 inches), and preferably at most 12mm (0.47 inches), and more preferably 6 mm (0.24 inches), and mostpreferably 4.5 mm (0.18 inches). In an embodiment the thickness of theAlMgSc-series aluminium rolled product is at least 1.2 mm (0.047inches).

In a particular embodiment the AlMgSc-series aluminium alloy product isin the form of an extruded product.

In an embodiment the AlMgSc-series aluminium alloy rolled product iscast, subsequently rolled to final gauge and annealed. The alloy can beprovided as an ingot or slab for fabrication into rolling feedstockusing casting techniques regular in the art for cast products, e.g.Direct Chill DC-casting, and preferably having an ingot thickness in arange of about 220 mm or more, e.g. 400 mm, 500 mm or 600 mm. In anotherembodiment thin gauge slabs resulting from continuous casting, e.g. beltcasters or roll casters, also may be used, and having a thickness of upto about 40 mm. After casting the rolling feedstock, the thick as-castingot is commonly scalped to remove segregation zones near the castsurface of the ingot.

Preferably the rolling process applied comprises hot rolling, andoptionally comprises hot rolling followed by cold rolling to finalgauge, and where applicable an intermediate annealing is applied eitherbefore the cold rolling operation or during the cold rolling operationat an intermediate cold rolling gauge.

Prior to hot rolling the AlMgSc-series aluminium alloy product ishomogenised or pre-heated for up to about 50 hours, preferably up toabout 24 hours, at a temperature in a range of about 320° C. to 470° C.,preferably of about 320° C. to 450° C.

In an embodiment following the hot rolling operation the hot rolledproduct receives a very mild cold rolling step (skin rolling or skinpass) with a reduction of less than about 1%, preferably less than about0.5%, to improve the flatness of the rolled product. In an alternativeembodiment the hot rolled product can be stretched. This stretching stepcan be carried out with a reduction of up to 3%, preferably betweenabout 0.5% to 1%, to improve the flatness of the hot rolled product.

The final annealing or annealing heat-treatment at final gauge is torecover the microstructure and is typically performed at a set annealingtemperature in the range of 250° C. to 400° C., preferably in the rangeof 260° C. to 375° C., and more preferably in a range of about 280° C.to 350° C., for a time in a range of about 0.5 hours to 20 hours, andpreferably of about 0.5 hours to 10 hours.

In an embodiment the AlMgSc-series aluminium alloy extruded product isproduced by a method comprises the steps, in that order, of: (a)providing an extrusion ingot, e.g. by means of DC-casting, of thealuminium alloy as herein described and claimed; (b) preheating and/orhomogenisation of the extrusion ingot; preferably at temperature andtimes similar as for the rolling feedstock; (c) hot extruding the ingotinto an extruded profile having a section or wall thickness in a rangeof 1 mm to about 20 mm, preferably 1 mm to about 15 mm; the billettemperature at the start of the extrusion process is typically in arange of about 400° C. to about 500° C.; optionally stretching of theextruded profile to increase product straightness, and (d) annealing ofthe extruded profile at a final annealing temperature followed by thecooling procedure in accordance with the present invention.

In an embodiment of the invention the method of cooling the aluminiumalloy product is applied immediately following a high-temperatureforming operation for shaping the AlMgSc-series aluminium alloy productinto a single- or double-curved shape product. The high-temperatureforming operation is performed at the final annealing temperature in therange of 180° C. to 500° C., preferably in the range of 250° C. to 400°C., more preferably in a range of 260° C. to 375° C., and mostpreferably in a range of 280° C. to 350° C., for example at about 300°C. or at about 325° C. A particular preferred embodiment of such ahigh-temperature forming operation at the final annealing temperature isby means of a creep forming operation or a relaxation forming operation.Creep forming is a process or operation of restraining a component to aspecific shape during heat treatment, allowing the component to relievestresses and creep to contour, for example fuselage shells with a doublecurvature. This creep forming process is for example explained in thepaper by S. Jambu et al., “Creep forming of AlMgSc alloys for aeronauticand space applications”, published at the occasion of the ICAS-2002congress.

In a preferred embodiment of the high-temperature forming operation atthe final annealing temperature into a single- or double-curved shapeproduct a rolled AlMgSc-series aluminium alloy product is beingemployed. The AlMgSc-series aluminium alloy product can be provided inan annealed condition also manufactured by the method according to thisinvention.

Optionally also extruded AlMgSc-series aluminium alloy products arebeing employed, for example as extruded stringers as part of a fuselagepanel.

In an embodiment of the invention the method of cooling the aluminiumalloy product is applied on a welded product or panel incorporating theAlMgSc-series aluminium alloy product immediately following a post-weldheat-treatment to recover some strength in particular by reprecipitatingAlScZr dispersoids. The post-weld heat-treatment is performed at atemperature similar as for the final anneal heat-treatment and is in therange of 250° C. to 400° C., preferably in the range of 260° C. to 375°C., and more preferably in a range of about 260° C. to 350° C., for atime in a range of about 0.5 hours to 20 hours, and preferably of about0.5 hours to 10 hours.

In an embodiment of the invention the method of cooling the aluminiumalloy product is applied on a cold-formed and shaped product from theAlMgSc-series aluminium alloy whereby an annealing heat-treatment isperformed to reduce residual stress in the cold-formed and shapedproduct or to recover certain engineering properties such as elongationor damage tolerance. Such an annealing heat-treatment is performed at atemperature similar as for the final anneal heat-treatment and is in therange of 250° C. to 400° C., preferably in the range of 260° C. to 375°C., and more preferably in a range of about 280° C. to 350° C., for atime in a range of about 0.5 hours to 20 hours, and preferably of about0.5 hours to 10 hours.

In an embodiment the AlMgSc-series aluminium alloy has a compositioncomprising, in wt. %:

Mg 3.0% to 6.0%, preferably 3.2%-4.8%, more preferably 3.5% to 4.5%,

Sc 0.02% to 0.5%, preferably 0.02%-0.40%, more preferably 0.05%-0.3%,

Mn up to 1%, preferably 0.3% to 1.0%, more preferably 0.3% to 0.8%,

Zr up to 0.3%, preferably 0.05% to 0.3%, more preferably 0.07% to 0.15%,

Cr up to 0.3%, preferably 0.02% to 0.2%,

Ti up to 0.2%, preferably 0.01% to 0.2%,

Cu up to 0.2%, preferably up to 0.1%, more preferably up to 0.05%,

Zn up to 1.5%, preferably up to 0.8%, more preferably 0.1% to 0.8%,

Fe up to 0.4%, preferably up to 0.3%, more preferably up to 0.20%,

Si up to 0.3%, preferably up to 0.2%, more preferably up to 0.1%,

impurities and balance aluminium. Typically, such impurities are presenteach <0.05% and total <0.15%.

The Mg is the main alloying element in the AlMgSc-series alloys, and forthe method according to this invention it should be in a range of 3.0%to 6.0%. A preferred lower-limit for the Mg-content is about 3.2%, morepreferably about 3.8%. A preferred upper-limit for the Mg-content isabout 4.8%. In an embodiment the upper-limit for the Mg-content is about4.5%.

Sc is another important alloying element and should be present in arange of 0.02% to 0.5%. A preferred lower-limit for the Sc-content isabout 0.05%, and more preferably about 0.1%. In an embodiment theSc-content is up to about 0.4%, and preferably up to about 0.3%.

Mn may be added to the AlMgSc-series aluminium alloys and may be presentin a range of up to about 1%. In an embodiment the Mn-content is in arange of about 0.3% to 1%, and preferably about 0.3% to 0.8%.

To make Sc more effective, it is preferred to add also Zr in a range ofup to about 0.3%. In an embodiment the Zr is present in a range of 0.05%to 0.30%, preferably in a range of about 0.05% to 0.25%, and morepreferably is present in a range of about 0.07% to 0.15%.

Cr can be present in a range of up to about 0.3%. When purposively addedit is preferably in a range of about 0.02% to 0.3%, and more preferablyin a range of about 0.05% to 0.15%. In an embodiment there is nopurposive addition of Cr and it can be present up to 0.05%, andpreferably is kept below 0.02%.

Ti may be added up to about 0.2% to the AlMgSc alloy as strengtheningelement or for improving the corrosion resistance or for grain refinerpurposes. A preferred addition of Ti is in a range of about 0.01% to0.2%, and preferably in a range of about 0.01% to 0.10%.

In an embodiment there is a purposive combined addition of Zr+Cr+Ti. Inthis embodiment the combined addition is at least 0.15% to achievesufficient strength, and preferably does not exceed 0.30% to avoid theformation of too large precipitates.

In another embodiment there is a purposive combined addition of Zr andTi but no purposive addition of Cr. In this embodiment the combinedaddition of Zr+Ti is at least 0.08%, and preferably does not exceed0.25%, and wherein Cr is up to 0.02%, and preferably only up to 0.01%.

Zinc (Zn) in a range of up to 1.5% can be purposively added to furtherenhance the strength in the aluminium alloy product. A preferred lowerlimit for the purposive Zn addition would be 0.1%. A preferred upperlimit would be about 0.8%, and more preferably 0.5%, to provide abalance in strength and corrosion resistance.

In an embodiment the Zn is tolerable impurity element and it can bepresent up to 0.15%, and preferably up to 0.10%.

Cu can be present in the AlMgSc-alloy as strengthening element in arange up to about 2%. However, in applications of the product where thecorrosion resistance is a very critical engineering property, it ispreferred to maintain the Cu at a low level of about 0.2% or less, andpreferably at a level of about 0.1% or less, and more preferably at alevel of 0.05% or less. In an embodiment the Cu-content is 0.01% orless.

Fe is a regular impurity in aluminium alloys and can be tolerated up toabout 0.4%. Preferably it is kept to a level of up to about 0.3%, andmore preferably up to about 0.20%.

Si is also a regular impurity in aluminium alloys and can be toleratedup to about 0.3%. Preferably it is kept to a level of up to 0.2%, andmore preferably up to 0.10%.

In an embodiment the AlMgSc-series aluminium alloy has a compositionconsisting of, in wt. %: Mg 3.0% to 6.0%, Sc 0.02% to 0.5%, Mn up to 1%,Zr up to 0.3%, Cr up to 0.3%, Ti up to 0.2%, Cu up to 0.2%, Zn up to1.5%, Fe up to 0.4%, Si up to 0.3%, balance aluminium and impuritieseach <0.05% and total <0.15%, and with preferred narrower compositionalranges as herein described and claimed.

In accordance with the invention it has been found that the method canbe employed to a wide range of AlMgSc-series aluminium alloys. It hasbeen found that with increasing Cu-content in the aluminium alloy alower cooling rate and thus a longer equivalent time in the definedfirst and second temperature range from the final annealing temperatureis being preferred. Such a very low cooling rate has no adverse effecton the corrosion performance of AlMgSc-series aluminium alloys having avery low Cu-content, for example less than 0.05%, or even less than0.01%.

In an embodiment the aluminium alloy product is a single or doublecurved panel, in particular a single or double curved aircraft fuselagepanel.

The invention will now be illustrated with reference to the followingnon-limiting example, both according to the invention and comparative.

Example

Sheet products of 4.5 mm have been manufactured on an industrial scalecomprising the steps of DC-casting of a rolling ingot, scalping,milling, preheating to hot rolling temperature between 400° C. and 450°C., hot rolling, cold rolling to 4.5 mm and with intermediate annealingduring the cold rolling operation, and final annealing at a settemperature of 325° C. (598K) for 2 hours and followed by differentcontrolled cooling rates according to Table 1 and whereby specimen A, Band C are according to the invention, and specimen D is comparative.

The AlMgSc aluminium alloy cast has the following composition, in wt. %,4.0% Mg, 0.55% Mn, 0.2% Sc, 0.3% Zn, 0.1% Zr, 0.07% Cr, 0.07% Ti, 0.02%Si, 0.02% Fe, 0.006% Cu, balance aluminium and inevitable impurities.

Table 1 lists the measured mass loss per ASTM G67-13 for each specimenhaving different cooling regimes from the final annealing temperatureafter sensitising at 120° C. for 168 hours.

TABLE 1 cooling after final annealing at 598K between 523K-473K between473K-423K cooling equivalent cooling equivalent rate time t(eq) ratetime t(eq) mass loss specimen [K/h] [hrs] [K/h] [hrs] [mg/cm²] A 95 4.1560 0.22 8.7 B 60 6.58 30 0.43 7.4 C 15 26.31 15 0.87 6.0 D 60 6.58 aircooling in still air at RT 16.5

The AlMgSc-series aluminium alloy rolled product manufactured inaccordance with the invention are resistant to intergranular corrosion.“Resistant to intergranular corrosion” means that, both before and afterthe AlMgSc-series aluminium alloy has been sensitized, the aluminiumalloy product passes ASTM Standard G67-13, (NAMLT Test)”. All sensitizedspecimen had a PA performance, and all non-sensitized specimen had alsoa PA performance.

From the results of Table 1 it can be seen that the AlMgSc-seriesaluminium alloy rolled products manufactured in accordance with theinvention achieve a measured mass loss per ASTM G67-13 not greater than15 mg/cm² after being sensitized. The better examples have a mass lossnot greater than 9 mg/cm². With a slower cooling rate or a longerequivalent time in the defined temperature range the mass loss isfurther reduced. Specimen D had in the temperature of 473K to 423K a toofast cooling rate and outside the invention resulting in a significantincreased mass loss per ASTM G67-13.

Thus the method according to the invention results in an aluminium alloyproduct having a good intergranular corrosion resistance in combinationwith a good exfoliation corrosion resistance.

Similar corrosion performance of the aluminium alloy product will beachieved in the cooling down from a high-temperature forming operationperformed at the final annealing temperature, for example a creepforming operation performed at 310° C. or 325° C.

The invention is not limited to the embodiments described before, andwhich may be varied widely within the scope of the invention as definedby the appending claims.

1. A method of manufacturing an AlMgSc-series aluminium alloy product,the method comprising the step of cooling said AlMgSc-series aluminiumalloy product from a final annealing temperature to below 150° C.,wherein the cooling in a first temperature range of about 250° C. toabout 200° C. is at an equivalent time of more than 4 hours, and whereinthe cooling in a second temperature range from about 200° C. to about150° C. is at an equivalent time of more than 0.2 hours, and wherein theequivalent time (t(eq)) is defined as${t({eq})} = \frac{\int{{\exp\left( {{- 16000}/T} \right)}{dt}}}{\exp\left( {{- 16000}/T_{ref}} \right)}$wherein T (in degrees Kelvin) indicates the temperature of the heattreatment, which changes over the time t (in hours) and T_(ref) (indegrees Kelvin) is the reference temperature selected at 473K (200° C.).2. The method according to claim 1, wherein the equivalent time in thefirst temperature range is longer than 6.5 hours.
 3. The methodaccording to claim 1, wherein the equivalent time in the secondtemperature range is longer than 0.4 hours.
 4. The method according toclaim 1, wherein the final annealing temperature is in a range of 250°C. to 400° C.
 5. The method according to claim 1, wherein saidAlMgSc-series aluminium alloy product is in a form selected from thegroup consisting of a rolled product, an extruded product, a forgedproduct, a powder-metallurgy product.
 6. The method according to claim1, wherein said AlMgSc-series aluminium alloy product is a rolledproduct.
 7. The method according to claim 6, wherein the rolled producthas a thickness of up to 25.4 mm.
 8. The method according to claim 1,wherein the method comprises the steps of casting an AlMgSc-seriesaluminium alloy ingot, rolling the ingot to final gauge into a rolledproduct, and heat-treating by annealing of the rolled product at thefinal annealing temperature, followed by cooling in accordance withclaim
 1. 9. The method according to claim 1, wherein the methodcomprises the steps of a high temperature forming operation of anAlMgSc-series aluminium alloy product into a single- or double-curvedshape product at the final annealing temperature followed by cooling inaccordance with claim
 1. 10. The method according to claim 9, whereinthe high temperature forming operation at the final annealingtemperature is by a creep forming operation or a relaxation formingoperation.
 11. The method according to claim 1, wherein theAlMgSc-series aluminium alloy product has a composition comprising of,in wt. %: Mg 3.0% to 6.0%, Sc 0.02% to 0.5%, Mn up to 1%, Zr up to 0.3%,impurities and balance aluminium.
 12. The method according to claim 1,wherein the AlMgSc-series aluminium alloy product has a compositioncomprising of, in wt. %: Mg 3.0% to 6.0%, Sc 0.02% to 0.5%, Mn up to 1%,Zr up to 0.3%, Cr up to 0.3%, Ti up to 0.2%, Cu up to 0.2%, Zn up to1.5%, Fe up to 0.4%, Si up to 0.3%, impurities and balance aluminium.13. The method according to claim 1, wherein the AlMgSc-series aluminiumalloy product achieves a measured mass loss per ASTM G67-13 not greaterthan 15 mg/cm², both before and after being sensitized.
 14. The methodaccording to claim 13, wherein the measured mass loss is not greaterthan 12 mg/cm².
 15. The method according to claim 13, wherein themeasured mass loss is not greater than 9 mg/cm².
 16. The methodaccording to claim 12, wherein the Mg content is from 3.2% to 4.8%. 17.The method according to claim 12, wherein the Sc content is from 0.02%to 0.40%.
 18. The method according to claim 12, wherein the Mn contentis from 0.3% to 1.0%.
 19. The method according to claim 12, wherein theZr content is from 0.05% to 0.3%.
 20. The method according to claim 12,wherein the Ti content is from 0.01% to 0.2%.